Aequorin, the Ca-sensitive photoprotein, will be confined in a dialysis tube at the center of the squid axon and its light emission measured with a photomultiplier. The photo current will be used as an indicator of Ca net flux as previous experiments have shown that light emission increases if the axon is in seawater in 10 mM Ca and decreases if Ca is 1 mM or less. The relative ability of different external monovalent cations to maintain zero new flux of Ca will be examined by substituting them for the (Na) of seawater. Further studies using metabolic inhibitors to decrease ATP levels in axons will allow us to evaluate the importance of ATP in the operation of Ca transport system. BIBLIOGRAPHIC REFERENCES: The Sensitivity of Calcium Efflux from Squid Axons to Changes in Membrane Potential. L.J. Mullins and F.J. Brinley, Jr. Journal of General Physiology, 1975, 65:135-152. Calcium and EDTA Fluxes in Dialyzed Squid Axons. F.J. Brinley, Jr., S.G. Spangler, and L. J. Mullins. Journal of General Physiology, 1975, 66:223-250.